where \(p_1\)  is the dipole moment of the first atom, and \(p_2\)  is the dipole moment of the second atom. \(C_{k,q}\)  is a spherical tensor [1]}. \(R\)  is the distance between the two quadrupoles. On-resonance dipole-dipole interactions [2]}, which are proportional to \(\frac{1}{R^3}\) , may also cause broadening. However, the on-resonance dipole-dipole coupling is caused by the dipole between the 5p atoms and the 34d atoms in the case studied, and the size of this broadening is negligible due to the big size difference between the 5p atoms and the Rydberg atoms, 34d atoms.
